1. Field of the Invention
The present invention relates to a method and apparatus for detecting a supply voltage by means of the so-called zero-magnetic-flux method.
The present application is based on Japanese Patent Application No. Hei. 10-163990, which is incorporated herein by reference.
2. Description of the Related Art
The technology for detecting a voltage in a specific location of a circuit or a current flowing through the specific location in a non-contacting manner by utilization of the so-called zero-magnetic-flux method or zero-flux method has already been utilized in various fields. For example, Japanese Patent Application Laid-open No. Hei. 8-86813 discloses a circuit for detecting a voltage or current to be used for estimating the amount of current stored in the batteries of an electric car.
The principle of the zero-magnetic-flux method is described by reference to a circuit diagram which is shown in FIG. 2, which has been prepared by simplification of the circuit diagram referred to in Japanese Patent Application Laid-open No. Hei. 8-86813 in order to describe the related art.
As shown in FIG. 2, reference numeral 1 designates a toroidal core which assumes a substantially C-shaped form having a predetermined gap 1a; i.e., a magnetic core. A primary winding 3 and a secondary winding 5 are coiled around the magnetic core 1. In conjunction with the magnetic core 1, the primary winding 3 and the secondary winding 5 form a coil 4.
One end of an energization resistor 7A is connected to one end of the primary winding 3; the other end of the primary winding 3 forms a primary-side terminal 9a; and the other end of the energization resistor 7A forms another primary-side terminal 9b.
A hole element 11 is placed in the gap 1a of the magnetic core 1 for generating a voltage proportional to the magnetic flux developing in the magnetic core 1. After having been amplified by a differential amplifier circuit 13, the voltage output from the hole element 11 is delivered to a current buffer 15 formed by a push-pull circuit having an NPN transistor 15a and a PNP transistor 15b. The current buffer 15 supplies an electric current i.sub.2 in the direction from a terminal 5a of the secondary winding to another terminal 5b of the same.
The circuit shown in FIG. 2 forms a servo system, in which an electric current which flows through the secondary winding 5 corresponds to the electric current that flows through the primary winding 3.
In the circuit shown in FIG. 2, when an electric current i.sub.1, flows through the primary winding 3, a magnetic flux .phi..sub.1 corresponding to the electric current i.sub.1 develops in the magnetic core 1. Further, an output voltage corresponding to the current i.sub.1 also arises in the hole element 11. After having been amplified through differential amplification by means of the differential amplifier circuit 13, the output voltage is delivered to the current buffer 15. The electric current i.sub.2, which flows from the current buffer 15 in the direction from the terminal 5a of the secondary winding 5 to the terminal 5b of the same, generates a magnetic flux .phi..sub.2 in the magnetic core 1 which cancels the magnetic flux .phi..sub.1.
If this situation continues, the magnetic flux .phi..sub.2 converges so as to become equal in magnitude to the magnetic flux .phi..sub.1. Consequently, the magnetic core 1 reaches a magnetic equilibrium state, so that the voltage output from the hole element 11 assumes a value of zero.
As a result, the electric current that is supplied to the current buffer 15 after having been subjected to differential amplification in the differential amplifier circuit 13 decreases to zero, thereby diminishing the magnetic flux .phi. developing in the magnetic core 1. At the same instant, a voltage corresponding to the current i.sub.1 that flows through the primary winding 3 again develops in the hole element 11, so that the magnetic flux .phi..sub.2 again converges so as to become equal in magnitude to the magnetic flux .phi..sub.1. Consequently, the coil 4 again reaches the magnetic equilibrium state.
More specifically, the magnetic flux .phi..sub.2 always varies in the vicinity of the magnetic flux .phi..sub.1, and the coil 4 always remains in a magnetic equilibrium state. This is the so-called zero-magnetic-flux principle.
In the state in which the coil 4 is in a magnetic equilibrium state, a relationship existing between the electric current i.sub.1 that flows through the primary winding 3 and the electric current i.sub.2 that flows through the secondary winding 5 is expressed by i.sub.1 .times.N.sub.1 =i.sub.2 .times.N.sub.2, provided that the number of turns of the primary winding 3 is N.sub.1 and the number of turns of the secondary winding 5 is N.sub.2.
The invention described in Japanese Patent Application Laid-open No. Hei. 8-86813 is specifically intended to apply a single power supply to a circuit which detects a voltage or current in a non-contacting manner in order to estimate the charge stored in the batteries of an electric car, through utilization of the zero-magnetic-flux method. However, the above-described invention encounters another, new problem, as will be described below.
More specifically, in order to detect the voltage of the power supply through use of the circuitry shown in FIG. 2, as indicated by dashed lines in FIG. 2, a power supply to be detected 17 is connected to the primary-side terminal 9a, and the other primary-side terminal 9b is grounded by way of the energization resistor 7A. Further, the terminal 5b of the secondary winding 5 is grounded by way of a detection load 21, and a potential difference develops across the detection load 21. This potential difference is multiplied by a predetermined coefficient.
In the case of detecting the power supply 17 through use of the foregoing circuit configuration, there arises a problem--not unique to the batteries of the electric car--of the energization resistor 7A developing a large amount of heat from the electric current i.sub.1 flowing through the primary winding 3 due to the high voltage of the power supply 17, provided that the power supply 17 has a considerably high voltage. To prevent an increase in the amount of the heat developing in the energization resistor 7A, a resistor for use with high power (hereinafter called a "high-power resistor") must be used as the energization resistor 7A, thereby resulting in an increase in the size and cost of the circuitry.
Further, if the power supply 17 has a considerably high voltage, the rated capacity of the primary winding 3 must be increased, which in turn requires an increase in the value of resistance of the energization resistor 7A.
In contrast, if the resistance value of the energization resistor 7A is indiscriminately increased in order to increase the rated capacity of the primary winding 3, the magnetic flux .phi..sub.1 developing in the magnetic core 1 diminishes by an amount equal to a reduction in the amount of the electric current i.sub.1 flowing through the primary winding 3, thereby involving a reduction in the detection accuracy of the entire circuitry. Prevention of this reduction also raises another, new problem of a necessity for an increase in the number of turns N.sub.1 of the primary winding 3.
The present invention has been conceived in view of the foregoing problem, and the object of the present invention is three-fold, to provide a method and apparatus for detecting a supply voltage that: (1) prevents an increase in the size and cost of circuitry, which would otherwise be caused by using a high-power resistor as an energization load; (2) prevents a reduction in the accuracy of detection of the entire circuitry, which would otherwise be caused by increasing the value of resistance of the energization load in order to increase the rated capacity of the primary winding; and (3) can be used with a power source of a considerably high voltage.